There is known a water treatment device that cleans waste water that includes contaminants, such as sewage and plant waste water, using a membrane bioreactor. This water treatment device includes a biological treatment tank, an air diffuser, and a filtration membrane. The biological treatment tank stores activated sludge that contains microorganisms and waste water that flows into the tank. The air diffuser aerates the biological treatment tank by supplying an oxygen containing gas to the activated sludge. The aeration of the biological treatment tank causes the microorganisms to decompose the contaminants in the waste water, breed, and proliferate. The filtration membrane filters a suspension of the treated water and the activated sludge in the biological treatment tank, separating the treated water from the activated sludge. When the activated sludge is aerated, the filtration membrane is cleaned by an upflow generated by rising bubbles, preventing clogging (refer to Patent Documents 1 to 3).
There is known a two-phase high-load activated sludge system that separates microbiota in activated sludge into two phases, and treats water using the two phases. This two-phase high-load activated sludge system includes a first aeration tank and a second aeration tank. The first aeration tank treats untreated waste water using non-agglomerative bacteria only. The second aeration tank further treats the waste water treated by the first aeration tank using activated sludge having excellent protozoa and metazoa that feed upon the non-agglomerative bacteria. Such a two-phase high-load activated sludge system is capable of treating untreated waste water with high efficiency, making it possible to reduce the size of the tanks and decrease excess sludge.